1. Field of the Invention
The present invention relates to cable assemblies having an aluminum grounding conductor.
2. Description of Related Art
Metal clad (MC) and armored cables (AC), are a flexible metal conduit with a wire bundle, consisting of individually insulated conductors covered by a flexible spiral-wound interlocking layer of metal tape. Most often used for feeder and service power in commercial and multi-story residential applications, the steel or aluminum clad cables are more resistant to damage from puncture or abrasion than non-metallic sheathed cable.
Various standards across the world govern the specifications for metal clad (MC) and armored cables (AC). These standards include those specified by Underwriter Laboratories under UL1569 for metal clad (MC) cables, UL 4 for AC cables and CSA C22.2 No.51 for AC90 cables. Other standards include those promulgated by National Engineering Code (NEC) sections 330 governing MC cables and 250 governing grounding arrangements. For Non Metallic cables the codes set forth in NEC 334 apply as well. Additionally, when these types of cables are used in Healthcare facilities, other sections of NEC, such as NEC Article 517 provide further characteristic requirements for proper grounding when equipment interfaces with human patients and operators.
Furthermore, AC cables having a 90 degree insulation rating (AC90) are specified by Canadian Standards Association under CSA C22.2 No 51, the installation is covered by the Canadian Electrical Code (CEC) part 1 section 12-600 for installation and 10-600 for bonding methods. Rule 10-618 of the CEC covers termination.
Typically, MC cables that are constructed in accordance with the specification set forth by UL 1569 include a cabled core of at least two conductors along with an insulated copper bonding conductor cabled with the conductors. A printed plastic tape binder covers the conductors and bonding conductor assembly, with the resultant assembly encased within an aluminum interlock armor. The grounding copper conductor, which at the cable termination provides the grounding capacity required under the specified standards. In reality there are two paths for MC cables for grounding, through the armor strip and through the designated insulated grounding conductor.
Typically, AC cables contain a conductor assembly that includes at least three insulated conductors. One of the conductors is designated as a grounding conductor. Each insulated conductor is individually wrapped with a paper covering. A plastic legend tape is added and printed to identify the cable and manufacturer as specified in UL 4. The conductors are twisted together or cabled into a bundle in accordance with the lay requirements set forth in UL 4. A bonding strip is laid lengthwise along the core conductors while the entire assembly is subjected to an armoring process. The bonding strip may be made of a strip of a thin bare aluminum bonding wire disposed in direct contact with the metal-tape armor. There are three paths for AC cables for grounding, through the armor strip, through the bonding strip, and through the designated insulated grounding conductor.
AC90 cables are manufactured for the Canadian market (Armored Cable with 90 degree C. insulation) per CSA C22.2 No. 51 for Armoured Cables (Canadian Standard Association). Typically, MC or AC cables described above and made to UL 1569, and UL 4 do not meet the requirements of CSA C22.2 No. 51. AC90 cables are typically two, three or four copper or compact ACM aluminum conductors with low temperature cross-linked polyethylene insulation (R90 XLPE) to meet −40° C. Cold Bend and Cold Impact test requirements, PVC/Nylon insulation is sometimes used, but only meets −25° C. Cold Bend and Cold Impact test requirements. A bare copper or aluminum bonding wire is cabled in the assembly, and the assembly is paper wrapped and covered with a bare interlocking aluminum armor. In the circuit sizes 14 to 10 AWG, the conductors are typically solid, and for sizes 8 AWG and larger, the conductors are typically stranded.
NMD (Non Metallic sheathed) cables are constructed to CSA C22.2 No. 48 in Canada and called NMD90 (90° C. insulation) and UL 719 and called NMB in the USA. NMD90 typically is made in a 2 or 3 conductor arrangement. For the 2 conductor construction, the color coded insulated conductor is placed longitudinally and the bare grounding conductor is placed in the center and covered by a sheath, as shown in a figure 8 profile. Cable identification is printed on the cable. Sheaths are colored to help the installer distinguish the different conductor sizes. In the 3 conductor construction, the conductors with a bare grounding conductor are cabled and covered by a sheath. For the 10 AWG to 14 AWG circuit sizes, the conductors are typically solid, for sizes 8 AWG and larger, the conductors are stranded. Different insulation systems can be applied to meet the 90° C. temperature rating. For the NMB, manufactured to UL 719—Nonmelallic Sheathed Cables, in the 2 conductor construction, the bare grounding conductor is covered by a paper wrap, and the over-all assembly is given a paper wrap, and then sheathed. In the 3 conductor construction, only the bare grounding conductor is covered by a paper wrap and one has the option to cable the conductors in a parallel or twisted configuration. Insulation and sheath thicknesses are specified in the appropriate Standards. 8 AWG and larger conductors are typically stranded.
Recently, in view of the rising cost of copper, cable manufacturers have introduced a substitute design, wherein the MC cable contains a bare aluminum grounding conductor, which is cabled in concert with the conductor assembly in an arrangement where the conductors are encased within an insulating tape such as a polypropylene printed legend tape binder and the bare aluminum grounding conductor is cabled outside of the tape binder. For MC cables the grounding path of this cable now has combined the armor ground path and the bare ground path as one. Although this design has reduced the cost of manufacturing MC cables, there is still a need for alternative solutions and for providing a value better than that provided by products manufactured today.